BI how to use the Defocus node with images as plane
Well, as you put, the internal render doesn't see the alpha of the image it can only see the alpha of the object and this can't be changed. This is a common limitation of the internal renderer's depth of field. 

An often used workaround is to model with geometry the outline of whatever the texture is so that blender can see the alpha. For example if the texture was a leaf you would have to model the outline of the leaf before applying the texture to it. Of course, this might be time consuming for a complex texture but unfortunately that's one of the few workarounds. 

An alternative method is to render the plane with the texture onto a separate render layer and manually blur it in the compositor using a standard blur node and then layer it back onto the background.